1. Field of the Invention
The present invention discloses apparatus and methods for reducing visual interference patterns on cathode ray tube ("CRT") display devices. More particularly, the present invention discloses apparatus and methods for visually cancelling the Moire interference phenomena on color CRTs during the display of certain video signals comprised of alternating pixel patterns.
2. Art Background
A. Moire Background
Color cathode ray tubes ("CRTs") are commonly used as visual display devices, employing up to three electrodes, typically one for each primary color: red, green, and blue. Most color CRTs use a shadow mask to selectively illuminate a matrix of each electrode's respective colored phosphors (i.e., red, green, and blue). Referring briefly to FIG. 1, a CRT is shown with placement of a shadow mask behind a phosphor-coated screen. The shadow mask is usually a metal foil with numerous perforations which allow the electron beam sourced by a particular electrode to selectively strike its respective phosphor dot. The electron beam is focused by magnetic lenses in the CRT neck into a small spot before the electron beam reaches the shadow mask. The electron beam from the green cathode is partially occluded by the shadow mask such that the electron beam only strikes the corresponding green phosphor after passing through the shadow mask. The beam is typically larger than the shadow mask perforation size, so the shadow mask blocks part of the beam and casts a smaller shadow of the original beam onto the desired phosphor.
The dot pitch, or spacing, between adjacent shadow mask perforations, and their corresponding phosphor dots, must be as small as possible for the highest resolution. For mechanical and economic reasons, the dot pitch is generally limited to about 0.2 millimeters ("mm") to 0.3 mm for a typical high resolution display CRT. As the electron beam traverses the screen, the shadow mask includes a periodic illlumination pattern depending on whether the beam either impinges upon a perforation, and consequently the phosphor, or strikes the metal foil of the shadow mask separating the perforations. Because the sweep rate of the electron beam is known, an equivalent frequency for the resulting sinusoid can be calculated, and is referred to as the spatial frequency of the shadow mask, .nu..sub.spatial. The shadow mask spatial frequency is graphically illustrated in waveform 31 of FIG. 3, and will be explained in more detail in the discussion in connection with FIG. 3.
To increase the resolution of the display, the spot size of the incident electron beam must be made as small as possible. As the electron beam spot size is reduced and begins to approach the dimensions of the phosphor dot pitch, the amount of a particular phosphor that is actually struck by the beam is a function of how well the electron beam spot is aligned to the shadow mask aperture corresponding to the intended phosphor. Moreover, it must be noted that the electron beam spot shape is not constant as the beam traverses the CRT screen. In particular, the beam spot varies from a circular shape at small angles of deflection, e.g., near the center of the CRT screen, becoming more eccentric or ovaloid at higher angles of beam deflection, e.g., near the screen perimeter. If a video pattern of alternating on-off phosphors ("pixels") is displayed, some of the pixels will be seen to be exactly aligned with the shadow mask and therefore will have uniform phosphor brightness across the dot, whereas other phosphors will exhibit a nonuniform brightness, a consequence of misalignment between electron beam and shadow mask aperture. The repeating pattern of varyingly bright pixels also is seen to be of sinusoidal form, with a frequency .nu..sub.spot equivalent to half the pixel clock frequency, where one pixel clock cycle turns on the spot, and the next pixel clock cycle turns off the pixel. The pixel video and electron beam spot frequency is graphically illustrated in waveform 32 of FIG. 3, and will be explained in more detail in the discussion in connection with FIG. 3.
As the spot size of the electron beam is reduced while viewing the on-off pattern, a periodic visual interference pattern known as Moire is produced in each video line scanned across the CRT. The frequency .nu..sub.Moire of the Moire interference pattern is the difference between the spatial frequency of the shadow mask .nu..sub.spatial, and the frequency of the electron beam spot .nu..sub.spot, or EQU .nu..sub.Moire =.nu..sub.spatial -.nu..sub.spot.
The Moire frequency is graphically illustrated in waveform 33 of FIG. 3, and will also be explained in more detail in the discussion in connection with FIG. 3.
If the two frequencies .nu..sub.mask and .nu..sub.spot were identical and in-phase, the Moire frequency .nu..sub.Moire would zero. A Moire frequency of zero is the ideal case, where each phosphor has a corresponding shadow mask aperture through which the corresponding electron beam travels. From a particular standpoint, however, the spot size varies as a function of the electron beam deflection angle and focus voltage. Therefore, there may be a significant variation of electron beam spot size depending on the age of the CRT and position of the electron beam on the screen. Hence, the ideal case typically cannot practicably be realized. In fact, the closer the spatial frequency and the spot frequencies are to each other, the lower the Moire beat frequency .nu..sub.Moire and the more visible and objectionable the Moire interference pattern becomes. In addition, because the electron beam spot size varies across the face of the CRT, the individually scanned video lines will each produce a slightly different Moire interference, and therefore the Moire pattern itself varies as a function of electron beam position.
From an operating standpoint, the Moire interference phenomenon poses a serious aesthetic problem, since the best electron beam focus and highest image resolution results in unacceptably noticeable Moire patterns if the video signal being displayed includes alternating pixel patterns, which is a common occurrence. From the prior art teachings, the Moire interference problem has been addressed in three ways. First, the shadow mask and phosphor dot pitch can be reduced, which raises the effective spatial frequency of the CRT, thereby raising the Moire beat frequency so that it is less visible. The result is that in order to reduce the Moire effect, much lower resolution images must be displayed on a CRT which is inherently capable of significantly higher resolution. Second, the electron beam can be defocused so that the spot size of the electron beam is increased, thereby decreasing the amplitude of the phosphor illumination, which in turn reduces the amplitude of the phosphor spot frequency. The lower amplitude spot sinusoid results in a decrease of the amplitude, and therefore visibility, of the resulting Moire interference. Again, significant reduction in resolution and image quality are exchanged for only moderate reduction in Moire interference. A third option is to avoid displaying video signals with alternating pixel or phosphor illumination patterns, and to simply tolerate the resultant Moire interference patterns when they occur.
B. Hardware Multiplier Background
In conventional signal multipliers known in the art, AC signals are applied to two inputs and corresponding outputs are derived consisting of signals whose frequencies consist of the sum and difference of the two input signals. For example, if one input were a 51 megahertz ("MHz") sine wave and the second input were a 50 MHz sine wave, the resulting output would be two sine waves, one with frequency 101 MHz, and the other with frequency 1 MHz. The phase of the output waveforms are directly related to the phases of the two input signals. For example, if the second input were shifted in phase by 45 degrees, the output signals would also shift by 45 degrees, even though the periods of the input and output signals are vastly different. The phase-shifting of input signals permits the introduction of a time-shift, or delay, in the output signals. In the case of the exemplary input frequencies given above, a 45 degree phase shift in the 50 MHz signal corresponds to a time shift of 2.5 nanoseconds. The resulting 1 MHz output signal, also phase-shifted by 45 degrees, corresponds to a time shift of 12.5 microseconds, a four decade increase in time delat. The phase-shifting "multiplier-effect" AC multipliers can be used to good advantage in producing large output phase shifts for small differences in input phase.
As will be desfribed, the present invention overcomes the problem of Moire interference in color CRTs without sacrificing resolution or brightness of the displayed image.